The invention relates to a picture pick-up device provided with an image sensor in the form of a charge transfer device, which image sensor comprises picture pick-up elements for converting incident radiation into electric charge and a charge transfer shift register for periodically shifting the charges under the control of clock pulses originating from a control signal generator to a sensor output terminal for the supply of a pulsatory picture signal with periodically occurring reset pulses and picture signal pulses, said picture pick-up device further comprising a series arrangement of at least a signal clamping circuit for at least a part of a reset pulse and a signal sampling circuit for at least a part of a picture signal pulse, said series arrangement being coupled to the sensor output terminal and being controlled from the control signal generator.
A picture pick-up device of this type is known from European Patent Application No. 0,140,292. The application describes that on the one hand the reset pulses in the pulsatory picture signal are beset at a reset level with high-frequency thermal noise originating from a conducting reset transistor which is present in the shift register at the output terminal and on the other hand lead to a low-frequency reset noise caused by the variable amplitude of the reset clock pulses obtained by crosstalk in the picture signal. Both noise sources lead to a deterioration of the signal-to-noise ratio of the picture signal at which particularly the low-frequency reset noise seriously perturbs the quality of a reproduced picture.
The so-called "correlated double sampling" is mentioned as a known solution to improve the signal-to-noise ratio. The reset pulse in the picture signal then has a reset level (with noise and interference) and a constant reference level deviating therefrom at which the signal clamping circuit is active. The said Patent Application states as disadvantages: the short period of time in which the reference level is present for the signal clamping operation, the crosstalk from the short-lasting clamping clock pulse to the picture signal and the deterioration of the signal-to-noise ratio in the event that high-frequency noise is present at the reference level.
A given construction of the signal sampling circuit for the picture signal is given as the own solution for the signal-to-noise ratio improvement. In this case this circuit comprises two switches with which during the presence of the picture signal pulses at the circuit input these pulses are passed on to the circuit output by the one switch whilst the other switch is open, and the one switch is open between the picture signal pulses whilst a reference voltage is supplied to the circuit output via the other switch. It appears that the proposed signal sampling circuit for the pulsatory picture signal simply passes the picture signal pulses without any influence. The same applies to noise and interference which may be present in the picture signal pulses, which interference is caused, for example, by clock pulse crosstalk upon shifting the charges in the charge transfer shift register. Such an interference may also be present in a picture signal for which the correlated double sampling is applied. The erratic signal variation obtained by the interference in the picture signal pulses leads to a picture signal pulse sampling with an obtained sampled value which is dependent on the instantaneous interference value.